Generally described, a metal-air cell includes one or more oxygen electrodes separated from a metallic anode by an aqueous electrolyte. A metal-air cell also can include one or more oxygen electrodes that cooperate with suspended metallic anode particles in a paste-like electrolyte. During operation of a metal-air cell, such as a zinc-air cell, oxygen from the ambient air and water from the electrolyte are converted at the oxygen electrode to form hydroxide ions. The zinc is then oxidized at the anode and reacts with the hydroxide ions such that water and electrons are released to provide electrical energy.
Metal-air cells have been recognized as a desirable means for powering portable electronic equipment, such as personal computers, camcorders, and telephones. As compared to conventional electrochemical power sources, metal-air cells provide relatively high power output and long lifetime with relatively low weight. These advantages are due in part to the fact that the metal-air cells utilize oxygen from the ambient air as the reactant in the electrochemical process as opposed to a heavier material such as a metal or a metallic composition.
Air managers have been developed that provide the metal-air cells with a flow of reactive air so as to support high power output while also isolating the cells from the ambient air and changes in humidity, particularly when no power output is required. For example, a mechanical air door system is shown in U.S. Pat. No. 4,913,983 to Chieky. This reference describes a fan used to supply a flow of ambient air to a pack of metal-air cells within the battery housing. When the battery pack is turned on, the mechanical air doors adjacent to an air inlet and an air outlet are opened and the fan is activated to create the flow of air into, through, and out of the housing. The air doors are then closed when the battery is turned off to isolate the cells from the environment. Although the mechanical air doors may limit the transfer of oxygen, water vapor, and contaminates into and out of the housing when the fan is off, such mechanical air doors add complexity to the battery housing itself and, inevitably, increase the size and cost of the overall battery pack.
A vast improvement in air manager technology is found in commonly owned U.S. Pat. No. 5,691,074 to Pedicini, entitled "Diffusion Controlled Air Vent for a Metal-Air Battery". Pedicini discloses, in one embodiment, a group of metal-air cells isolated from the ambient air except for an inlet and an outlet passageway. These passageways may be, for example, in the form of elongate tubes. An air-moving device, such as a fan, may be positioned within the housing to force air through the inlet and outlet passageways so as to circulate and to refresh the air across the oxygen electrodes. The passageways are sized to allow sufficient airflow therethrough while the air mover is operating but also to restrict the passage of water vapor therethrough while the passageways are unsealed and the air mover is not operating.
When the air mover is off and the humidity level within the cell is relatively constant, only a very limited amount of oxygen diffuses through the passageways. The water vapor within the cell largely protects the oxygen electrodes from exposure to oxygen. The oxygen electrodes are sufficiently isolated from the ambient air by the water vapor such that the cells have a long "shelf life" without sealing the passageways with a mechanical air door. These passageways may be referred to as "diffusion tubes", "isolating passageways", or "diffusion limiting passageways" due to their isolating capabilities.
Specifically, FIG. 1 herein shows one embodiment of the metal-air battery disclosed in Pedicini. The metal-air battery 10 includes a plurality of cells 15 enclosed within a housing 20. The housing 20 isolates the cells 15 from the ambient air with the exception of a plurality of ventilation openings 25. A single air inlet opening 30 and a single air outer opening 35 are utilized herein. A circulating fan 40 is provided for convective airflow both into and out of the housing 20 and to circulate and mix the gases within the housing 20. The arrows 45 shown in FIG. 1 represent a typical circulation of the gases into, out of, and within the housing 20 to provide the reactant air to the cells 15. The fan 40 forces the air through the air inlet 30, into an air plenum inlet 55, across the cells 15, out of an air plenum outlet 65, and then either to recirculate within the housing 20 or to pass out of the air outlet 35. U.S. Pat. No. 5,691,074 is incorporated herein by reference.
The isolating passageways act to minimize the detrimental impact of humidity on the metal-air cells, especially while the air-moving device is off. A metal-air cell that is exposed to ambient air having a high humidity level may absorb too much water through its oxygen electrode and fail due to a condition referred to as "flooding." Alternatively, a metal-air cell that is exposed to ambient air having a low humidity level may release too much water vapor from its electrolyte through the oxygen electrode and fail due to a condition referred to as "drying out." The isolating passageways limit the transfer of moisture into or out of the metal-air cells while the air mover is off, so that the negative impacts of the ambient humidity level are minimized.
The efficiency of the isolating passageways in terms of the transfer of air and water into and out of a metal-air cell can be described in terms of an "isolation ratio." The "isolation ratio" is the rate of the water loss or gain by the cell while its oxygen electrodes are fully exposed to the ambient air as compared to the rate of water loss or gain by a cell while its oxygen electrodes are isolated from the ambient air except through one or more limited openings. For example, given identical metal-air cells having electrolyte solutions of approximately thirty-five percent (35%) KOH in water, an internal relative humidity of approximately fifty percent (50%), ambient air having a relative humidity of approximately ten percent (10%), and no fan-forced circulation, the water loss from a cell having an oxygen electrode fully exposed to the ambient air should be more than one hundred (100) times greater than the water loss from a cell having an oxygen electrode that is isolated from the ambient air except through one or more isolating passageways of the type described above. In this example, an isolation ratio of more than about one hundred (100) to one (1) may be obtained.
In accordance with the above-referenced example from Pedicini, the isolating passageways also function to limit the amount of oxygen that can reach the oxygen electrodes when the fan is off and the internal humidity level is relatively constant. This isolation minimizes the self-discharge and leakage or drain current of the metal-air cells. Self-discharge can be characterized as a chemical reaction within a metal-air cell that does not provide a usable electric current. Self-discharge diminishes the capacity of the metal-air cell for providing a usable electric current. Self-discharge occurs, for example, when a metal-air cell dries out and the zinc anode is oxidized by the oxygen that seeps into the cell during periods of non-use. Leakage current, which is synonymous with drain current, can be characterized as the electric current that can be supplied to a closed circuit by a metal-air cell when oxygen is not provided to the cell by an air mover. The isolating passageways as described above may limit the total drain current to an amount smaller than the output current by a factor of at least about fifty (50) times.
Although the isolating passageways described above are effective in providing oxygen to the cells during periods of use and also in isolating the cells during the periods of non-use, the effectiveness of the battery as a whole may be compromised if the passageways are somehow obstructed. For example, the isolating passageways may be blocked if water, sand, dust, or other materials inadvertently fill or cover the passageways. Further, the passageways also may be blocked if the battery is placed against a non-air permeable surface. Once the isolating passageways are blocked, the cells will be inoperative due to a lack of oxygen. Further, the user may not even know if the passageways are clogged or how to remedy the problem.
There is a need, therefore, for a metal-air cell and/or battery pack that maintains the advantages of the isolating passageways while also ensuring an adequate air flow path. These advantages must be accomplished in a metal-air cell or battery pack that provides the traditional power and lifetime capabilities of a metal-air cell in a low cost, efficient manner.